Electrical transmission lines and power equipment used in heavy-duty power systems must be protected against fault conditions which could cause serious and expensive damage and personal injuries. Such a fault condition, for instance, might be a transmission line insulator failure which causes the transmission line to fall from an elevated position and to make contact with the ground. Line currents under such a condition can increase to several times the normal current and will destroy or damage both the lines and the attached equipment if the faulted line section is not cleared within a very short time after the fault condition occurs.
Conventionally, transmission lines are protected by electromechanical and electronic relaying systems which monitor the analog voltages and currents through the various lines through isolation transformers for the purpose of detecting out-of-tolerance operating conditions. Such systems process the line information in an analog format and achieve system reliability by having an independent protective relay dedicated to the protection of a single transmission line. This one-to-one correspondance between protection hardware and protected equipment, when considering the many transmission lines of a large system to be protected, yields the high reliability required by utilities but also results in relatively high equipment costs.
A major problem exists with conventional systems not in use. Modern remote control, automation and adaptive control techniques are being applied to power systems at all levels; however, currently used analog relay equipment cannot be conveniently interfaced with such systems. Settings and parameter modifications must generally be made by hand, rendering such analog systems useless for real time adaptive control. There have been no available digital relaying systems which can meet the strict reliability, security, and speed requirements of conventional systems, yet which offer a remotely adaptive capability.
Previous attempts at digital protective relay techniques have utilized single computer concepts wherein a medium-sized machine is used to monitor and protect transmission lines and related equipment. Such techniques have been described in the following references:
1. Rockefeller, G. D., "Fault Protection with a Digital Computer", Transmission IEEE, Vol. PAS-88, No. 4, April, 1969, pp. 438-61. PA1 2. Phadke, A. G., M. Ibrahim, and T. Hlibka, "Computer in an EHV Substation: Programming Considerations and Operating Experience", American Electric Power Service Corporation. PA1 3. Walker, Lewis, Steve Beottner and Granville Ott, "Software Considerations for Substation Computers", University of Missouri, Rolla. PA1 4. Mann, B. J. and I. F. Morrison, "Relaying a Three Phase Transmission Line with a Digital Computer", IEEE Transactions, Vol. PAS-90, No. 2, March/April, 1971.
Systems of the type described in the above references have proven the feasibility of digital information processing techniques for protective relaying; however, they have not been totally acceptable from a security and reliability standpoint. Because of these limitations, such systems have never been used by utilities to perform line protection.
In view of the foregoing, a need presently exists for a digital relaying system which can meet the strict reliability, security and speed requirements of conventional systems, yet offer a remotely adaptive capability for increased line protection.